Negative active material for lithium secondary battery and negative electrode and lithium secondary battery comprising same

ABSTRACT

A negative electrode for a lithium secondary battery of the present invention includes a negative active material including a plate-shaped carbon powder particle agglomerated in a primary structure in which the plate-shaped particle is oriented in a plane direction and laminated. Fine carbon powder particles are then agglomerated to form a secondary structure on the surface of the primary structure such that the resulting particles include fine pores formed on the surface.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/104,871, filed Apr. 12, 2005, which claims the benefit of andpriority to Korean Patent Application Nos. 10-2004-0024867 and10-2004-0025768 filed on Apr. 12, 2004 and Apr. 14, 2004, respectively,in the Korean Intellectual Property Office, the entire disclosures ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a negative active material for alithium secondary battery, and a negative electrode and a lithiumsecondary battery comprising the same. It more particularly relates to anegative active material for a lithium secondary battery having goodcompression strength, wettability for an electrolyte, and dischargecharacteristics at a low temperature, and that is capable of providing ahigh density electrode plate, and a negative electrode and a lithiumsecondary battery comprising the same.

BACKGROUND OF THE INVENTION

Lithium secondary batteries comprise a positive electrode and a negativeelectrode comprising materials capable of reversibly intercalatinglithium ions, with an organic electrolyte or a polymer electrolyteinjected between the positive electrode and the negative electrode. Thebattery produces electrical energy as a result of a redox reaction uponintercalating/deintercalating the lithium ions.

The positive active material for the lithium secondary battery employs achalcogenide compound such as a metal composite compound of LiCoO₂,LiMnO₂, LiMn₂O₄, LiNiO₂, or LiNi_(1-x)Co_(x)O₂(where 0<x<1).

The negative active material typically comprises a lithium metal, but abattery short circuit may occur in a battery using such a material asthe negative active material due to dendrite formation. Therefore, thelithium metal may be substituted with a carbonaceous material.Crystalline carbon such as natural graphite or synthetic graphite andamorphous carbon such as soft carbon or hard carbon have been mentionedfor this purpose. While amorphous carbon has a very large capacity, itmay cause problems in that it is hard to reverse intercalation duringthe charge and the discharge. Crystalline carbon, when used for thenegative active material, is generally in the form of a natural graphitesince it has a high capacity with a theoretical limit capacity of 372 mAh/g, but it has a problem in that the cycle life of the resultantbattery is remarkably degenerated.

The negative active material for a negative electrode for a lithiumsecondary battery is prepared by mixing carbon materials and a binder,and a conductive agent if required, and agitating them to provide aslurry. Then, the slurry is coated on a metal current collector anddried to form a negative electrode. The negative active material isfurther compressed onto the current collector in order to obtain auniformly thick electrode plate and increase the capacity of theelectrode plate. However, the carbon material tends to crack uponcompressing the electrode plate, so the electrode plate may lose itsuniformity, which causes the reaction with the electrolyte to loseuniformity and the life of the electrolyte plate to degenerate.Furthermore, if a crack develops, an edge portion thereof is exposed bythe pulverized crack so as to increase a side reaction with theelectrolyte, and the viscosity of the electrolyte is remarkablyincreased at a low temperature such as −20° C. because the electrolyteis soaked into the fine cracks by capillary action and the electrolytetaking part in the battery reaction is decreased. Thereby, the dischargecharacteristic at the low temperature is degenerated.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a negative active materialis provided for a lithium secondary battery capable of providing anelectrode plate having a high density and a good compression strength ofthe particle, a negative electrode, and a lithium secondary batterycomprising the same.

In an embodiment of the present invention, a negative active materialcomprises a plurality of negative active material particles, eachincluding a plurality of plate-shaped carbon powder particlesagglomerated in a primary structure in which the plate-shaped particlesare oriented in a plane direction and laminated to one another with finepores formed on the surface of the negative active material particle.

In still another embodiment of the present invention, a negativeelectrode for a lithium secondary battery comprises such a negativeactive material.

In yet another embodiment of the present invention, a lithium secondarybattery is provided comprising the negative electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view showing a structure of the negative activematerial for the negative electrode plate according to the presentinvention;

FIG. 2 is a view showing one embodiment of the lithium secondarybattery; and

FIG. 3 is a picture showing a cross-section of a negative electrodefabricated using the negative active material according to Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown in FIG. 1, according to an embodiment of the invention, anegative active material is provided that comprises a plurality ofnegative active material particles 10. Each negative active materialparticle comprises a plurality of plate-shaped carbon powder particles20 agglomerated in a primary structure in which the plate-shaped carbonpowder particles are oriented in a plane direction and laminated to oneanother. Fine pores are formed on the surface of the plate-shaped carbonpowder particles. Additional fine carbon powder particles 30 may beagglomerated to form a secondary structure on the surface of theplate-shaped carbon powder particles.

The plate-shaped carbon powder particles are formed in a primarystructure in which the plate-shaped carbon powder particles are orientedin a plane direction and laminated to one another, whereby each particleis reflected by a different polarization. The negative active materialparticles are preferably of a spherical or pseudo-spherical shape. Theplate-shaped carbon powder particles are of natural or syntheticgraphite with an average particle size of 10 μm or more, and morepreferably between 15 and 25 μm. The plate-shaped carbon powderparticles preferably have an intensity ratio of the (110) plane/(002)plane of 0.2 or less, and more preferably 0.002 to 0.2, determined byX-ray diffraction.

The fine carbon powder particles are pulverized particles generatedwhile pulverizing the plate-shaped carbon powder particles, and have anaverage length or an average particle size of 5 μm or less, andpreferably between 0.5 and 3 μm. The fine carbon powder particles may beformed as fibers, amorphous shapes, flake shapes, or other shapes. Theaverage particle size is measured using a particle size analyzer(Marvern.2000) after dispersing the carbon powder particles in solventusing ultra-sonication.

The fine carbon powder particles may have an amorphous or a turbostraticstructure in part or as a whole. The fine carbon powder particles areformed into an agglomerated body on the plate-shaped carbon powderparticles to obtain a high density negative active material having finepores on its surface. As a result of the fine pores, the absolute amountof conservation of electrolyte is increased, and thereby the high ratecharacteristics and the low temperature characteristics are improved.According to the prior art, when the fine pores are present in the wholeof the active material, it is easily compressed upon preparing a highdensity electrode plate and the specific surface of the active materialin the electrode plate is increased, causing new edge faces to beprojected which brings about a side reaction with the electrolyte.However, according to the present invention, as the pores are presentonly on the surface of the negative active material particles, they arenot easily broken even during compression in forming an electrode plate,and the shapes of the negative active material particles are maintained.Thereby, side reactions with the electrolyte are prevented and anelectrode plate having good swelling suppression characteristics may befabricated.

Referring to FIG. 1, the carbon powder particle may further comprise anamorphous carbon coating layer 40 other than the fine carbon powderparticle layer on the surface thereof. In one embodiment of theinvention, the amorphous carbon coating layer 40 is present in a rangeof 5 to 30 parts by weight, and preferably 5 to 15 parts by weight basedon 100 parts by weight of the carbon powder particle.

The negative active material particles according to one embodiment ofthe present invention have ratios of major axes to minor axes of 4 orless, preferably from 1 to 3. In an embodiment of the invention, thefine porosity ratio of the negative active material occupied by thepores having a diameter of 0.5 μm or less is 10% to 40%, preferably 15%to 40%, and more preferably 10% to 35%. In another embodiment of theinvention, the fine porosity ratio of the negative active materialoccupied by the pores having diameter of 0.1 μcm or less is 12% or less,and preferably from 3% to 10%.

Even though the fine porosity of the electrode plate is generallyreduced more than that of the active material during the compressionstep of the electrode plate, the fine porosity of the active materialaccording to the present invention is 2.0 times or less, preferably 1.1to 1.5 times that of the electrode plate showing that the fine porosityof the active material is almost not changed after the compression step.

The pores having a diameter of 10 to 100 μm act to improve the speed ofabsorption of the electrolyte and the speed of transporting the lithiumions in the electrode plate applied with the negative active material.If the electrode plate is formed of a conventional carbon material, theporosity area occupied by the pores of 10 to 100 μm is remarkably less.In the present invention, since the porosity area occupied by the poreshaving a diameter of 10 to 100 μm is 30 to 60%, preferably 35 to 50%, itis advantageous with respect to conserving the electrolyte, assuring thefluid pathway, and improving the lithium ion transport and the lowtemperature discharge characteristics.

According to an embodiment of the present invention, the negative activematerial is a high density carbon material having a tap density of 1.0g/cc or more, preferably 1.1 to 1.30 g/cc, and an appearance density of0.6 to 1.0 g/cc. Further, the ratio of the tap density to the appearancedensity is 30% or more.

According to an embodiment of the invention, the specific areadetermined by the BET method ranges from 2.0 m²/g to 4.0 m²/g. As thecompression strength of the negative active material is 15 MPa or more,and preferably from 15 to 45 MPa, the active material is rarely damagedduring the compression step of the electrode plate.

Hereinafter, the method of producing the negative active material isdescribed in detail.

Initially, edge parts or protruded parts of the carbon material areremoved by a first mechanic dynamic pulverizing process to obtain aspherical or a pseudo-spherical material. The carbon material may be anatural graphite, a synthetic graphite, a graphite precursor, or similarmaterials. The graphite precursor may be a graphite carbon fiber, agraphite mesocarbon microbead (MCMB), or similar materials.

During the first mechanical-dynamic pulverization process, as the carbonmaterial is formed into the spherical or the pseudo-spherical shapedparticles, the plate-shaped carbon powder particles are obtained. Duringthis process, the fine carbon powder particles are also obtained. At thesame time, small cracks are generated between crystalline granuleshaving a certain directional orientation inside the carbon material, andthereby the orientation of crystalline granule is totally shifted. Also,fluidity among the crystalline granules is ensured by generating thecracks because of fine particle impurities present therein.

According to an embodiment of the present invention, the carbon materialis subsequently agglomerated with the fine carbon powder particles via asecond mechanical-dynamic pulverizing process by applying a very strongfriction force and a shearing force between surfaces thereof. During thesecond agglomerating process, the plate-shaped carbon powder particlesare oriented in a plane direction and laminated. By the very strongfriction stress between surfaces, the Van der Waals force between planesurfaces is maximized and interlocking is maximally generated. Theplate-shaped carbon powder particles are then dried and agglomerated toform a multi-laminated spherical or pseudo-spherical powder. The finecarbon powder particles are secondary-agglomerated on the surface of theplate-shaped carbon powder particles in a structure such that the finecarbon powder particles coat the surface of the plate-shaped carbonpowder particles. At this time, the fine carbon powder particles areagglomerated in a random direction while forming fine pores. Theagglomerating processes according to the present invention are carriedout in a dry atmosphere.

The mechanical-dynamic pulverizing process utilizes forces such ascompression, impact, shear, and friction. The pulverizing processgenerally utilizes all of these four forces, and is not limited to aparticular force or forces, but it is dependant upon the designcharacteristics or the driving conditions of the device. The smoothingprocess on the particle surface is carried out by the first mechanicoperation mainly using a force generated by compression and impact.Representative examples of pulverization devices include rotor mills,ACM mills, pin mills, and jet mills. For the second agglomeration andattachment of the fine carbon powder particles to the surface, thedevice applying the shearing force and the compression friction forcemay implement mechanofusion, hybridization, and so on.

The second agglomerating process may further comprise the step of addingthe fine carbon powder particles. Further, the second agglomeratingprocess may comprise the step of adding an amorphous carbon precursor toagglomerate the fine carbon powder particles with the plate-shapedcarbon powder particles. Suitable amorphous carbon precursors includemesophase pitch, petroleum-based pitch, carboniferous pitch, and heavygrade oil having a low molecular weight. Such amorphous carbonprecursors may be added in an amount of 5 to 30 parts by weight based on100 parts by weight of carbon materials.

The agglomerated body obtained from the aforementioned processes issubjected to heat treatment to provide a negative active materialaccording to the present invention. The heat treatment process iscarried out at 1000° C. or more, preferably 1200 to 2400° C. If theheating treatment is carried out at less than 1000° C., the impurityhetero atoms are not completely removed.

According to one embodiment of the present invention, a negativeelectrode plate is provided comprising the above-described negativeactive material. The negative electrode plate is obtained by coating aslurry comprising a negative active material and a binder resin on ametal current collector, and drying and compressing the same.

As the negative active material according to the present invention has ahigh density and a high compression intensity, it is possible to obtaina high density electrode plate of 1.5 g/cc to 2.0 g/cc, preferably 1.8to 2.0 g/cc. According to the present invention, even though such a highdensity electrode plate is applied to the battery, it is possible toprovide a high capacity battery since sufficient pathways for absorbingthe electrolyte are maintained.

The binder resin can be selected from any binder resin used for aconventional lithium secondary battery. Examples include polyvinylidenefluoride, carboxymethyl cellulose, methyl cellulose, and sodiumpolyacrylic acid. The metal current collector may include a punchingmetal, a mesh punching metal, gold foil, a metal foam, a net metal fibersinter, a nickel foil, or a copper foil.

Further, the negative electrode according to an embodiment of thepresent invention may further comprise a conductive agent such as anickel powder, cobalt oxide, titanium oxide, or carbon. The carbon mayinclude ketjen black, acetylene black, furnace black, graphite, carbonfiber, or fullerene.

According to another embodiment of the present invention, the lithiumsecondary battery comprises the aforementioned negative electrode plate.The lithium secondary battery may comprise negative and positiveelectrodes, an electrolyte, and if required, a separator.

The positive electrode of the lithium secondary battery may be anypositive electrode used for a conventional lithium secondary battery,e.g., one prepared by mixing a positive active material powder with abinder and a conductive agent and forming it into a paste or a sheetshape.

Suitable positive active materials are those selected from the groupconsisting of LiMn₂O₄, LiCoO₂, LiNiO₂, LiFeO₂, and V₂O₅. In anembodiment of the invention, it is preferred that the positive activematerial be capable of intercalating lithium. Examples of such materialsinclude TiS, MoS, organic disulfide compounds, or organic polysulfidecompound. Examples of the conductive material include ketjen black,acetylene black, furnace black, graphite, carbon fiber, and fullerene.Suitable binders include polyvinylidene fluoride, carboxymethylcellulose, methylcellulose, sodium polyacrylic acid.

A positive electrode is obtained by mixing a slurry comprising positiveactive material powder, a binder, and a conductive agent, coating anddrying it on the metal current collector, and compressing it to form adesired shape.

The separator may be any single- or multi-layer conventional separatorused for a lithium secondary battery, and it may include, for example,polyethylene, polypropylene, polyvinylidene fluoride, polyamide, andglass fiber.

The electrolyte for the lithium secondary battery may include anyorganic electrolyte in which a lithium salt is dissolved in anon-aqueous solvent.

The non-aqueous solvent may include ethylene carbonate, propylenecarbonate, butylene carbonate, benzonitrile, acetonitrile,tetrahydrofuran, 2-methyltetrahydrofuran, butyrolactone, dioxolane,4-methyldioxolane, N,N-dimethylformamide, dimethylacetoamide,dimethylsulfoxide, dioxane, 1,2-dimethoxy ethane, sulforane,dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate,methylethyl carbonate, diethyl carbonate, methylpropyl carbonate,methylisopropyl carbonate, ethylbutyl carbonate, dipropyl carbonate,diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethylether, and mixtures thereof. Further, it may include any conventionalsolvent used for a lithium secondary battery. It is preferably a mixtureof any one of propylene carbonate, ethylene carbonate, and butylcarbonate with any one of dimethyl carbonate, methylethyl carbonate, anddiethyl carbonate.

The lithium salt may include, but is not limited to, one or a mixture oftwo or more selected from the group consisting of LiPF₆, LiBF₄, LiSbF₆,LiAsF₆, LiCF₃SO₃, Li(CF₃SO₂)₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiClO₄, LiAlO₄,LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (where x and y arenatural numbers), LiCl, and Lil.

Further, the electrolyte may include a polymer electrolyte comprisingthe organic electrolyte and a polymer such as polyethylene oxide,polypropylene oxide, polyacetonitrile, polyvinylidene fluoride,polymethacrylate, or polymethylmethacrylate, which add good swellingproperties to the organic electrolyte.

A lithium secondary battery is fabricated by inserting the negativeelectrode, the positive electrode, the electrolyte, and if required, theseparator into the battery case. FIG. 2 shows a perspective view of thelithium secondary battery 1 according to one embodiment of the presentinvention. The lithium secondary battery 1 is formed in a cylindricalshape and is mainly composed of a negative electrode 2; a positiveelectrode 3; a separator 4 placed between the negative electrode 2 andthe positive electrode 3; an electrolyte injected in the negativeelectrode 2, the positive electrode 3, and the separator 4; acylindrical battery container 5; and a sealing member 6 sealing thebattery container 5. In the lithium secondary battery 1, the negativeelectrode 2, the positive electrode 3, and the separator 4 are laminatedin order, then wound in a spiral shape and inserted in the batterycontainer 5.

The following examples further illustrate the present invention indetail, but are not to be construed to limit the scope thereof.

EXAMPLES AND COMPARATIVE EXAMPLES Example 1

Plate-shaped graphite powder particles having an average diameter of 30μm were introduced into a pin mill and preliminarily pulverized. Theprojected and edge parts of the plate-shaped graphite powder particleswere removed to form an agglomerated body having a spherical or apseudo-spherical shape. The resulting plate-shaped graphite powderparticles were then introduced to a mechanofusion process and furtherpulverized. The plate-shaped graphite powder particles were oriented ina plane direction and laminated, and then fine graphite powder particleswere placed on the surface of the plate-shaped graphite powder particlesto obtain a second agglomerated carbon material that was heat treated at1000° C.

The resulting negative active material was mixed with 6 parts by weightof polyvinylidenefluoride (PVdF) as a binder and dispersed inN-methylpyrrolidone to produce a slurry for a negative electrode. Theslurry was coated on a copper current collector and compressed by a rollpress, thereby manufacturing a negative electrode having an electrodeplate density of 1.5 g/cc, 1.6 g/cc, 1.65 g/cc, 1.85 g/cc, and 2.0 g/cc.

Using the negative electrode and lithium metal as a counter electrode,coin-type half cells were fabricated. A mixed solution of 1.0 M LiPF₆dissolved in ethylene carbonate, dimethyl carbonate and ethyl methylcarbonate (in a 3/3/4 volume ratio) was used for the electrolyte.

Example 2

A cell was prepared in the same manner as in Example 1, except that thesecond agglomerated body was obtained by adding 10 parts by weight ofmesophase pitch based on 100 parts by weight of the agglomerated bodyobtained in Example 1.

Comparative Example 1

A cell was prepared in the same manner as in Example 1, except that thenegative active material was obtained by mixing natural graphite powderparticles, pitch, and a graphitizing catalyst having an average diameterof 15 μm with silica, and shaping the mixture, pulverizing the mixture,and carbonizing or graphitizing the resultant agglomerated amorphousparticles to produce graphite agglomerated particles as the negativeactive material.

Comparative Example 2

A cell was prepared in the same manner as in Example 1, except that thenegative active material was obtained by mixing carbonized cokeparticles, pitch, and a graphitizing catalyst with silica and shapingand pulverizing the mixture to an amorphous coarse body. Then, theamorphous particle was carbonized or graphitized by subjecting it to thehigh temperature treatment.

Comparative Example 3

A cell was prepared in the same manner as in Example 1, except that thenegative active material was amorphous artificial graphite. Compressionstrengths of the negative active materials according to Examples 1 and 2and Comparative Examples 1 to 3 were measured and the results are shownin Table 1. The compression strength was measured as follows: negativeactive material particles were layered in one layer and compressed byapplication of pressure and the point at which particle fractureoccurred was measured.

Further, the cells obtained from Examples 1 and 2 and ComparativeExamples 1 to 3 were charged to 0.2 C at room temperature a underconstant current and constant voltage condition. The initial dischargecapacity and charge-discharge efficiency according to electrode platedensity were measured. The results are shown in Table 1.

TABLE 1 Comparative Comparative Comparative Example 1 Example 2 Example1 Example 2 Example 3 Compression strength of 30 35 7 12 12 activematerial particles (MPa) initial discharge capacity 1.8 g/cc 350 mAh/g352 mAh/g 340 mAh/g 346 mAh/g 330 mAh/g according to electrode plate(92%) (94%) (92%) (92%) (92%) density (charge-discharge 2.0 g/cc 340mAh/g 348 mAh/g 310 mAh/g 324 mAh/g 315 mAh/g efficiency) (91%) (91%)(84%) (88%) (90%)

As shown in Table 1, the compression strength of the negative activematerial particles of Examples 1 and 2 was significantly improved overthose of Comparative Examples 1 to 3. Therefore, the active material ofExamples 1 and 2 ensure a sufficient penetration pathway of electrolytein a high density electrode plate. This is confirmed by the fact thatwhen comparing discharge capacity of cells including a high densityelectrode plate of over 1.8 g/cc, initial discharge capacities andefficiencies of Examples 1 and 2 were higher than those of ComparativeExamples 1 to 3.

The negative electrode plate of Example 1 was molded using an epoxyresin and cut, thereby facilitating inspection of a cross-sectionthereof. FIG. 3 is a photograph taken of the plate with an opticalmicroscope, showing one particle of the active material which indicatesvarious plate-shaped layers with circles.

Example 4

After mixing 94 parts by weight of LiCoO₂ as a positive active material,2 parts by weight of polyvinylidenefluoride (PVdF) as a binder, and 4parts by weight of super-P conductive agent, a positive slurry wasprepared by dispersing the mixture into N-methylpyrrolidone. The slurrywas coated on an aluminum foil and compressed with a roll press, therebymanufacturing a positive electrode having an active mass (activematerial layer) density of 3.4 g/cc.

A polyethylene separator was interposed between the positive electrodeprepared above and a negative electrode with an electrode plate densityof 1.60 g/cc as prepared in Example 1 to fabricate an electrode group.The manufactured electrode group was wound and pressed, then put into abattery case. An electrolyte was injected into the case to form abattery. A mixed solution of 1.0 M LiPF6 dissolved in ethylenecarbonate, dimethyl carbonate and ethyl methyl carbonate (in a volumeratio of 3/3/4) was used for the electrolyte.

Example 5

A cell was prepared in the same manner as in Example 4, except that thesecond agglomerated body was obtained by adding 10 parts by weight ofmesophase pitch based on the 100 parts by weight of the agglomeratedbody obtained in Example 4.

Comparative Example 4

A cell was prepared in the same manner as in Example 4, except that thenegative active material was obtained by mixing natural graphiteparticles, pitch, and a graphitizing catalyst having an average diameterof 15 μm with silica and shaping the mixture, pulverizing the mixture,and carbonizing or graphitizing the resultant agglomerated amorphousparticles to obtain a graphite agglomerated particles as the negativeactive material.

Comparative Example 5

A cell was prepared in the same manner as in Example 4, except that thenegative active material was obtained by mixing carbonized cokeparticles, pitch, and a graphitizing catalyst with silica and shapingand pulverizing the mixture to an amorphous coarse body. Then, theamorphous particles were carbonized or graphitized by subjecting them toa high temperature treatment.

Comparative Example 6

A cell was prepared in the same manner as in Example 4, except that thenegative active material was amorphous artificial graphite.

Mercury porosity of the negative active materials and the negativeelectrode plates according to Example 5 and Comparative Examples 4 to 6were determined with a porosimeter (manufactured by Micrometics Inc.),and the results are shown in Table 2. Further, the cells obtained fromExample 2 and Comparative Examples 1 to 3 were charged at 0.5 C at roomtemperature and discharged at 1 C at −20° C. to determine the dischargecharacteristics at low temperature. The discharge characteristics at lowtemperature shown in Table 2 are represented as the percent of dischargecapacity at a low temperature based on the discharge capacity at roomtemperature. The results are shown in the following Table 2.

TABLE 2 Compara- Compara- Compara- Exam- tive tive tive ple 5 Example 4Example 5 Example 6 Fine porosity (%) in active 25 7.0 10 10 materialparticles having voids of 0.5 μm or less Fine porosity (%) in 27 23 3612 electrode plate having voids of 0.5 μm or less Porosity (%) occupiedby 58 24 25 50 voids of 10 to 100 μm Discharge characteristic at 60% 30%40% 60% low temperature of −20° C.

As shown in Table 2, after compressing the negative electrodes accordingto Comparative Examples 4 and 5, it was found that the fine particlesagglomerating to one particle were broken to generate fine voids whilecarbon particles uniformly dispersed inside of the particle werecompressed, thereby, increasing the number of pores of 0.5 μm or less.Even though these voids had a fine porosity similar to that of Example5, the obtained electrode plate did not have uniformity from compressingthe active materials on the electrode plate, and consequently theelectrolyte reaction was also not uniform, causing the life span of theelectrode plate to be decreased. The 0.5 μm pores of the electrodeplates according to Comparative Examples 4 and 5 were newly formed onthe pulverized cracks derived from the compression process so that theedge faces of the graphite were exposed. Thereby, the side reaction wasincreased and the viscosity of the electrolyte was remarkably increasedin an environment such as at −20° C. Therefore, it became difficult togenerate a capillary phenomenon for the electrolyte in the fine cracksand it was difficult to cause the appropriate battery reaction. As aresult, the discharge characteristics at low temperature degenerated.However, the electrode plate of Example 5 had good fine porosity of 0.5μm or less of the active material and such fine porosity was wellmaintained. Further, the porosity of 10 to 100 μm is quite high so thatthe property of conserving electrolyte on the electrode plate accordingto Example 5 and the speed of transmitting the lithium ions wasexcellent so that the low temperature characteristics were shown to bequite good.

The negative active material of the present invention has a high densityand good particle compression strength, and provides a high densityelectrode plate and a high capacity battery. The inventive negativeelectrode of the lithium secondary battery applies the negative activematerial having fine pores developed only on the surface thereof.Therefore, it is rarely broken during the compression process,maintaining the shape of the active material and preventing sidereactions. It also imparts good swelling inhibition characteristics tothe battery. Further, the cycle-life characteristic and low temperaturedischarge characteristic are improved.

1. A method of preparing a negative active material for a lithiumsecondary battery comprising: mechanically pulverizing a carbon materialand shaping the pulverized carbon material into a spherical or apseudo-spherical shape while forming a plurality of fine carbon powderparticles; agglomerating the fine carbon powder particles on the carbonmaterial to provide an agglomerated particle; and heating theagglomerated particle to provide a negative active material.
 2. Themethod of preparing a negative electrode for a lithium secondary batteryaccording to claim 1, further comprising the step of adding an amorphouscarbon precursor during the agglomeration step.
 3. The method ofpreparing a negative electrode for a lithium secondary battery accordingto claim 1, wherein the amorphous carbon precursor is added in an amountof 5 to 30 parts by weight based on 100 parts by weight of carbonmaterial.
 4. A lithium secondary battery comprising a positive electrodecomprising a positive active material, a negative electrode including anegative active material, and an electrolyte comprising a lithium saltand a non-aqueous solvent, wherein the negative active materialcomprises: a plurality of plate-shaped carbon powder particlesagglomerated in a primary structure in which the plate-shaped carbonparticles are oriented in a plane direction and laminated; and finepores formed on the surface of the carbon powder particles.